1. Field of the Invention
The present invention relates to a velocity control method for a synchronous ac servo motor,. more particularly, which a system in the motor is controlled by an acceleration control mode and a feedforward control mode in systems with severe external load torque and inertia, such as robots, computer ,numerical control machine tools and the like, to thereby attain a rapid response characteristic and to minimize the effects from the variation of the load torque and inertia.
2. Description of the Prior Art
Recently, synchronous ac servo motors have been widely employed in industries requiring high performance, such as industrial robots, CNC machine tools and/or the like.
In an industrial robot, however, load torque and inertia are varied due to the variations of the position and the moving speed of the robot arm and the weight of objects to be grasped by the robot, and, hence, an ac servo motor control unit of respective joints of the robot is disadvantageously affected due to the aforementioned variations. Accordingly, there is a problem normal operation of that the robot is likely to be interfered.
Therefore, an ac servo motor control unit has been developed which is prinimally affected by the variation of the parameters, for example, load torque and inertia.
For example, a dc motor is controlled by using an acceleration control method instead of an ordinary current control method in order to embody a powerful control unit. But, since an ac servo motor must be supplied with a three-phase sine wave current unlike to the dc motor, the current control method cannot be subsituted for the acceleration control method in the ac servo motor. Also, the acceleration control represents a powerful property against the variation of the load torque, but it is slow in a response velocity.
Referring to FIG. 1 (a), there is shown a conventional PID (Proportional Integral Differential) controller disclosed in U.S patent publication No. 4,509,003.
Now, the operation of the conventional PID controller shown in FIG. 1 (a) will be described.
Firstly, if the velocity command Wr is supplied to the controller, the command Wr is differentiated by means of a differential controller 1 and supplied to a summer G1. Consequently, the summer G1 compares the velocity command Wr with the actual velocity data W of a motor (not shown) to produce a deviation value A between the data Wr and the data W. The deviation value A is amplified by a PI controller 2 and the amplified value A is then supplied to a summer G2. As a result, the summer G2 produces the control which, in turn, is supplied to an object to be controlled. The PID controller is simply shown in FIG. 1 (b).
With the conventional PID control method as described above however; it is difficult to organically adjust a proportional-controlled gain, an integral-controlled gain and a differential-controlled gain. As a result, there are problems in that the response characteristic is very deteriorated and it may be affected seriously according to the variation of the parameters, for example, load torque and inertia.